I'm going to do this in reverse order of how I started the American League post,
because I want to focus on a key point in the battle between Jason Heyward and Buster
Posey: playing time and position.

Jason Heyward, I don't have to tell you, was Atlanta's everyday right-fielder this
year, racking up 140 games in the outfield and 623 trips to the plate. The only thing
standing between Heyward and a full complement of games played was a disabled list trip
from June 27th to July 15th because of a left thumb bruise. (Injury data via the
spreadsheet compiled by Jeff Zimmerman.) Buster Posey, by contrast, did not grace
Giants fans with his presence on the major league roster until May 29th. He was the
team's everyday first basemean until the end of June, when a spot was finally cleared
for him to catch, which he did with aplomb through the end of the season. The final
tally was 76 games behind the plate and 30 on the right side of the infield. His total
plate appearances thus came to just 443, or 180 fewer than Heyward.

What do these position and playing time stats mean in terms of value? Posey's credit
for playing time comes to 12 runs, while his credit for position stands at just +3, a
number weighed down by the negative figure associated with his monthlong sojourn on the
infield. Heyward, as a corner outfielder, gets dinged on the positional adjustment
(-7), but his additional playing time adds up to +17. Before we even consider
performance, then (to the extent being on the field isn't "performance", which
of course it is), the tally stands at: Posey +15; Heyward +10.

Playing time isn't just important in figuring replacement level, though -- 180 extra
PAs are a lot for a hitter of Heyward's caliber in which to create additional value.
Heyward notched an excellent .393/.456 OBP/SLG line, exactly the kind of performance
that is likely to be undervalued by some fans for being heavy in on-base value and
(relatively) light in slugging. Heyward also added eleven steals, but he was caught
six times, so his pilfery was not a positive overall. Still, that .393 OBP is massive
(4th in the entire league), and it drove a batting runs value of +27 per Rally (or +28
per Fangraphs; or +40 per StatCorner).

The shape of Buster Posey's performance was more pleasing to power addicts: .357/.505.
Compared to Heyward, in other words, he traded 35 points of on-base for 50 points of
slugging. Unfortunately for Posey, that ratio is out of whack: OBP, without accounting
for lineup position, is something like 50-75% more important than SLG. This explains
why Posey has a higher OPS but lower wOBA than Heyward does -- wOBA actually measures
run-scoring value with the bat using empirical data, while OPS merely approximates it
by adding together two numbers willy-nilly. Anyway, Posey's contributions with the
wood come to +16, eleven runs fewer than Heyward, leaving him six runs behind before
considering defense and base-running.

I've already mentioned Heyward's poor work on steals, but the Baseball Prospectus
baserunning reports show that his overall running value made up for that, as he clocked
in at +2.4 for the year (leading the Braves, actually, which says more about the Braves
than it does Heyward). Buster Posey, while young and seemingly somewhat athletic, is
still a catcher, and he runs like one: he managed to take 2.3 runs off the board with
his legs this season (despite being on base less often than Heyward as a percentage and
due to playing time). Posey, then, would have to make up about a ten-run gap on
defense.

I don't have my prospect books with me here in my hotel, but my understanding is that
Heyward is a well-regarded defensive player. As such, unless Posey is an otherworldly
defensive catcher, I find it implausible that Posey could have made up a win's worth of
value in a half-season behind the plate. Jason Heyward, then, gets my NL Rookie of the
Year vote.

I should give at least passing mention to a couple of other NL rookies who had very
good seasons, but who were overshadowed by the Heyward-Posey duel. Neil Walker had a
+11 bat in 469 PAs as the Pirate second baseman, a very nice year. Dan Hudson notched
2.0 WAR in just 11 starts for the Diamondbacks (and his WAR in systems that use actual
RA is significantly higher, due to his 1.69 ERA (vs. a still-excellent 3.22 FIP)).
Saving the best honorable mention for last, Jaime Garcia's nearly full season in the
Cardinals' rotation (28 starts, though they babied him to just 163.3 innings) with a
3.41 FIP resulted in 3.2 WAR, a season that could stand up to Heyward's if you're a
non-believer in the Brave's defense in right field, though it'd take losing about 3/4
of a win with the glove to make this a discussion. That's certainly reasonable in the
range of fielding skill.